Glasses having improved toughness and scratch resistance

ABSTRACT

A silicate glass that is tough and scratch resistant. The toughness is increased by minimizing the number of non-bridging oxygen atoms in the glass. In one embodiment, the silicate glass is an aluminoborosilicate glass in which −15 mol %≦(R 2 O+R′O—Al 2 O 3 —ZrO 2 )—B 2 O 3 ≦4 mol %, where R is one of Li, Na, K, Rb, and Cs, and R′ is one of Mg, Ca, Sr, and Ba.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/277,573, filed on Nov. 25, 2008, the contents of which is relied uponand incorporated herein by reference in their entirety, and the benefitof priority under 35 U.S.C. §120 is hereby claimed, which claims thebenefit of priority under 35 U.S.C. §119(e) of U.S. ProvisionalApplication Ser. No. 61/004,677, filed on Nov. 29, 2007.

BACKGROUND

The invention relates to silicate glasses. More particularly, theinvention relates to silicate glasses in which the content ofnon-bridging oxygen atoms in the glass is minimized. Even moreparticularly, the invention relates to such silicate glasses that arescratch resistant and less susceptible to edge chipping.

Glass has the drawback of being brittle. Brittleness is generally isunderstood to be the ratio of the hardness divided of the glass to itsfracture toughness. Such brittleness leads to breakage, defects, andedge chipping, all of which are particularly problematic in applicationssuch as cover plates for mobile electronic devices, touch screens, watchcrystals, solar concentrators, windows, screens, containers, and thelike. Glass compositions having higher toughness are less brittle,resist crack propagation, and are less prone to other types of damage,such as edge chipping. Softer (i.e., less hard) glasses are lessbrittle, but are less scratch resistant.

SUMMARY

The present invention provides silicate glasses with improved resistanceto scratching and edge chipping. The glasses not only maximizetoughness, but allow maximization of hardness without impactingbrittleness. In, addition, the glasses are ion exchangeable anddown-drawable. The glasses may be used for cover glasses for mobileelectronic devices, touch screens, watch crystals, solar concentrators,windows, screens, and containers, as well as other application thatrequire a strong, tough glass.

Accordingly, one aspect of the invention is to provide a silicate glass.The silicate glass comprises at least one of alumina and boron oxide,and at least one of an alkali metal oxide and an alkali earth metaloxide, wherein −15 mol %≦(R₂O+R′O−Al₂O₃−ZrO₂)−B₂O₃≦4 mol %, where R isone of Li, Na, K, Rb, and Cs, and R′ is one of Mg, Ca, Sr, and Ba.

A second aspect of the invention is to provide an aluminoborosilicateglass. The aluminosilicate glass has a toughness in a range from about0.7 MPa m^(0.5) up to about 1.2 MPa m^(0.5) and brittleness of less thanabout 8.5 μm^(-0.5).

A third aspect of the invention is to provide a silicate glass. Thesilicate glass comprises: at least one of alumina and boron oxide; andat least one of an alkali metal oxide and an alkali earth metal oxide,wherein −15 mol %≦(R₂O+R′O−Al₂O₃−ZrO₂)−B₂O₃≦4 mol %, where R is one ofLi, Na, K, Rb, and Cs, and R′ is one of Mg, Ca, Sr, and Ba, and whereinthe silicate glass has a toughness in a range from about 0.7 MPa m^(0.5)up to about 1.2 MPa m^(0.5) and a brittleness of less than about 8.5μm^(0.5).

A fourth aspect of the invention is to provide a cover plate for anelectronic device. The cover plate comprises a silicate glass. Thesilicate glass comprises at least one of alumina and boron oxide and atleast one of an alkali metal oxide and an alkali earth metal oxide,wherein −15 mol %≦(R₂O+R′O−Al₂O₃−ZrO₂)−B₂O₃≦4 mol %, where R is one ofLi, Na, K, Rb, and Cs, and R′ is one of Mg, Ca, Sr, and Ba.

A fifth aspect of the invention is to provide an aluminoborosilicateglass. The aluminoborosilicate glass comprises a plurality ofnon-bridging oxygen atoms (NBOs), wherein the plurality of non-bridgingoxygen atoms NBO is given by NBO=R₂O+R′O−Al₂O₃−ZrO₂, wherein R₂O, R′O,Al₂O₃, and ZrO₂ are expressed in mol %, wherein R is one of Li, Na, K,Rb, and Cs, and R′ is one of Mg, Ca, Sr, and Ba, and wherein −15 mol %NBO−B₂O₃ 3 mol %.

These and other aspects, advantages, and salient features of the presentinvention will become apparent from the following detailed description,the accompanying drawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a plot of hardness, toughness and brittleness of glasscompositions as a function of the difference in the concentration orcontent of excess modifiers R₂O (R=Li, Na, K, Rb, Cs) and R′O (R′=Mg,Ca, Sr, Ba), and boron oxide (B₂O₃).

DETAILED DESCRIPTION

In the following description, like reference characters designate likeor corresponding parts throughout the several views shown in thefigures. It is also understood that, unless otherwise specified, termssuch as “top,” “bottom,” “outward,” “inward,” and the like are words ofconvenience and are not to be construed as limiting terms. In addition,whenever a group is described as comprising at least one of a group ofelements and combinations thereof, it is understood that the group maycomprise any number of those elements recited, either individually or incombination with each other. Similarly, whenever a group is described asconsisting of at least one of a group of elements or combinationsthereof, it is understood that the group may consist of any number ofthose elements recited, either individually or in combination with eachother. Unless otherwise specified, a range of values, when recited,includes both the upper and lower limits of the range.

Referring to FIG. 1, it will be understood that the illustration isintended for the purpose of describing a particular embodiment of theinvention and is not intended to limit the invention thereto.

Glass has the drawback of being brittle. Brittleness leads to theproblems of breakage, defects, and edge chipping. The brittleness of amaterial is often referred to in the art as the ratio of the hardness ofthe material to its fracture toughness. Glasses having a higher degreeof toughness are therefore less brittle and have a greater resistance tocrack propagation and scratching.

The present invention provides a silicate glass having high toughnessand lower brittleness. The silicate glass comprises at least one ofalumina (Al₂O₃) and boron oxide (B₂O₃) and, in one embodiment, comprisesboth alumina and boron oxide. In addition, the silicate glass comprisesat least one of an alkali metal oxide (also referred to herein as an“alkali oxide”) having the general formula R₂O and an alkali earth metaloxide (also referred to herein as an “alkali earth oxide”) having thegeneral formula R′O, where −15 mol %≦(R₂O+R′O−Al₂O₃−ZrO₂)−B₂O₃≦4 mol %,wherein R₂O, R′O, Al₂O₃, and ZrO₂ represent the amounts orconcentrations of the respective oxides expressed in mol %. In oneembodiment, −5 mol %≦(R₂O+R′O−Al₂O₃−ZrO₂)−B₂O₃≦4 mol %. Unless otherwisespecified, the term “alkali metal” refers to the Group IA metals lithium(Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs), andthe term “alkali earth metal” refers to the Group IIA metals magnesium(Mg), calcium (Ca), strontium (Sr), and barium (Ba). Unless otherwisespecified, the concentrations of the components of the glass areexpressed in mole percent (mol %).

The silicate glasses described herein are glasses in which non-bridgingoxygen atoms (NBOs) have been minimized to increase the toughness of theglass. The number of NBOs can be approximately described by the equationNBO=R₂O+R′O−Al₂O₃−B₂O₃−ZrO₂, wherein the amounts or concentrations ofR₂O, R′O, Al₂O₃, and ZrO₂ are expressed in mole percent (mol %), andwhere R is an alkali metal (Li, Na, K, Rb, Cs) and R′ is an alkali earthmetal (Mg, Ca, Sr, Ba). The presence of more NBOs typically decreasesthe melting temperature of the glass, thus facilitating melting,especially without the use of toxic fining agents such as As₂O₃ andSb₂O₃. Such modifier-rich glasses (i.e., glasses having relatively highconcentrations of R₂O and R′O) also have low toughness and high hardnessand are hence quite brittle.

In one embodiment, the silicate glass does not comprise B₂O₃ (i.e.,B₂O₃=0 mol %) and −15 mol %≦(R₂O+R′O−Al₂O₃−ZrO₂) 4 mol %. In anotherembodiment, −5 mol %≦(R₂O+R′O−Al₂O₃−ZrO₂) 4 mol %. In these instances,the number—or concentration—of NBOs is minimized by balancing the alkaliand alkali earth oxides with alumina and zirconia alone.

In another embodiment, the addition of B₂O₃ to these modifier-richglasses associates with—or “ties up”—the NBOs with B³⁺ ions. The NBOsare converted to bridging oxygen atoms through formation of BO₄tetrahedra, resulting in a glass that is tougher and still easy to melt.Thus, to minimize the NBO level, the optimal amount of B₂O₃ that shouldbe present in a glass should be approximately equal to the concentrationof excess modifiers in the glass—i.e., B₂O₃=Excessmodifiers=R₂O+R′O−Al₂O₃−ZrO₂.

While the hardness increases linearly with increasing excess modifier,the toughness goes through a maximum near 0, which is the point at whichthe excess modifier content is equal to the B₂O₃ content and the NBOsare consumed in BO₄ tetrahedra. Since the toughness is at a maximum near(R₂O+R′O−Al₂O₃−ZrO2)−B₂O₃=0, the brittleness is desirably minimized.Such glasses are much more resistant to damage, scratching, and edgechipping than those glasses with higher degrees of brittleness.

Glasses having extra B₂O₃ create trigonal BO₃ groups rather thantetrahedral BO₄ groups. In FIG. 1, hardness (line (1) in FIG. 1,expressed in GPa), toughness (line (2) in FIG. 1, expressed in MPam^(0.5)), and brittleness (line (3) in FIG. 1, expressed in μm^(0.5))are plotted as functions of excess modifiers minus B₂O₃ (i.e.,(R₂O+R′O−Al₂O₃−ZrO₂)−B₂O₃). As seen in FIG. 1, the hardness increaseslinearly with increasing excess modifier, while the toughness goesthrough a maximum near 0, at which point the excess modifier content isequal to the B₂O₃ content and the NBOs are consumed in BO₄ tetrahedra.Since the toughness is at a maximum near (R₂O+R′O−Al₂O₃−ZrO₂)−B₂O₃=0,the brittleness is desirably minimized, yielding glass that is much morerobust against damage and edge chipping. The presence of excess B₂O₃(i.e., (R₂O+R′O−Al₂O₃−ZrO₂)−B₂O₃<0 mol %)—and thus low NBO levels—yieldsa glass having a higher level of toughness. In contrast, the presence ofexcess modifiers (i.e., (R₂O+R′O−Al₂O₃−ZrO₂)−B₂O₃>0 mol %) yieldsglasses having lower levels of toughness, due to the larger number ofNBOs that are formed by alkali and alkali earth metal oxides. Inaddition, glasses having higher levels of modifiers have greaterhardness. The combination of lower toughness and higher hardness of highNBO glasses results in high brittleness values, which is undesirable.

In one embodiment, the silicate glass has a toughness in a range fromabout 0.7 MPa m^(0.5) up to about 1.2 MPa m^(0.5). In anotherembodiment, the toughness is in a range from about 0.8 MPa m^(0.5) up toabout 1.0 MPa m^(0.5).

In one embodiment, the silicate glass has a hardness in a range fromabout 6 GPa up to about 8 GPa. As previously described, brittle ness isdefined as the ratio of toughness to hardness. In one embodiment, thesilicate glass has a brittleness of less than about 8.5 μm^(−0.5). Inanother embodiment, the silicate glass has a brittleness of less thanabout 7 μm^(−0.5).

Glasses in which B₂O₃=NBO=R₂O+R′O−Al₂O₃−ZrO₂ may also be made with verylow liquidus temperatures (Table I) and have very high liquidusviscosities. As used herein, the term “liquidus viscosity” refers to theviscosity of a molten glass at the liquidus temperature, wherein theliquidus temperature refers to the temperature at which the very lastcrystals melt away as temperature is increased from room temperature.These properties permit these silicate glasses to be down-drawable;i.e., the glass is capable of being formed into sheets using down-drawmethods such as, but not limited to, fusion draw and slot draw methodsthat are known to those skilled in the art. Such down-draw processes areused in the large-scale manufacture of ion-exchangeable flat glass. Inone embodiment, the liquidus viscosity is at least 100 kilopoise(kpoise). In another embodiment, the liquidus viscosity is at least 160kpoise, and, in a third embodiment, the liquidus viscosity is at least220 kpoise.

The fusion draw process uses a drawing tank that has a channel foraccepting molten glass raw material. The channel has weirs that are openat the top along the length of the channel on both sides of the channel.When the channel fills with molten material, the molten glass overflowsthe weirs. Due to gravity, the molten glass flows down the outsidesurfaces of the drawing tank. These outside surfaces extend down andinwardly so that they join at an edge below the drawing tank. The twoflowing glass surfaces join at this edge to fuse and form a singleflowing sheet. The fusion draw method offers the advantage that, sincethe two glass films flowing over the channel fuse together, neitheroutside surface of the resulting glass sheet comes in contact with anypart of the apparatus. Thus, the surface properties of the glass sheetare not affected by such contact.

The slot draw method is distinct from the fusion draw method. Here themolten raw material glass is provided to a drawing tank. The bottom ofthe drawing tank has an open slot with a nozzle that extends the lengthof the slot. The molten glass flows through the slot/nozzle and is drawndownward as a continuous sheet therethrough and into an annealingregion. Compared to the fusion draw process, the slot draw processprovides a thinner sheet, as only a single sheet is drawn through theslot, rather than two sheets being fused together, as in the fusiondown-draw process.

The silicate glass comprises 62-70 mol. % SiO₂; 0-18 mol % Al₂O₃; 0-10mol % B₂O₃; 0-15 mol % Li₂O; 0-20 mol % Na₂O; 0-18 mol % K₂O; 0-17 mol %MgO; 0-18 mol % CaO; and 0-5 mol % ZrO₂, wherein 14 mol %≦R₂O+R′O≦25 mol%, where R is Li, Na, K, Cs, or Rb, and R′ is Mg, Ca, Ba, or Sr; 10 mol%≦Al₂O₃+B₂O₃+ZrO₂≦30 mol %; and −15 mol %≦(R₂O+R′O−Al₂O₃−ZrO₂)−B₂O₃≦4mol %. Other oxides such as, but not limited to, ZnO, SnO₂, Sb₂O₃,As₂O₃, La₂O₃, Y₂O₃, Fe₂O₃, and the like may also be individually addedto the glass in amounts less than 5 mol %. In one embodiment, thesilicate glass is an aluminoborosilicate glass: i.e., the glasscomprises both alumina and boron oxide. In one embodiment, thealuminoborosilicate glass comprises: 64-66 mol % SiO₂; 8-12 mol % Al₂O₃;1-11 mol % B₂O₃; 0-5 mol % Li₂O; 6-12 mol % Na₂O; 1-4 mol % K₂O; 0-4 mol% MgO; 0-6 mol % CaO; and 0-2 mol % ZrO₂, where 20 mol %≦R₂O+R′O≦24 mol%; and 16 mol %≦Al₂O₃+B₂O₃+ZrO₂≦29 mol %. Table 1 lists non-limitingexamples of silicate glass compositions of the present invention andtheir properties.

TABLE 1 196 1 2 3 4 5 6 7 SiO2 65.75 65.43 65.11 65.23 64.80 64.48 64.16Al2O3 11.33 10.84 10.36 11.1 9.87 9.39 8.90 Li2O 4.57 3.66 2.74 2.3 1.830.91 0.00 Na2O 6.22 7.66 9.10 9.805 10.53 11.97 13.41 K2O 1.11 1.75 2.402.715 3.04 3.69 4.33 MgO 0.00 0.74 1.49 2.41 2.23 2.98 3.72 CaO 0.000.32 0.65 0.125 0.97 1.30 1.62 B2O3 11.01 9.01 7.01 6.165 5.00 3.00 1.00ZrO2 0.00 0.40 0.80 0 1.20 1.60 2.00 SnO2 0.25 0.25 0.25 0.15 0.25 0.250.25 TOTAL 100.24 100.07 99.90 100.00 99.73 99.56 99.39 R2O +RO—Al52O3—ZrO2—B2O3 −10.44 −6.116 −1.792 0.09 2.532 6.856 11.18 GPa E66.4 71.8 74.7 76.1 75.2 74.8 GPa VH₁₀₀₀ 5.70 6.71 6.23 6.39 6.76 6.707.37 μm^(−0.5) B₁₀₀₀ 6.8 7.8 6.8 6.9 8.5 10.6 12.5 Strain (° C.) 486 481489 489 501 517 546 Anneal (° C.) 536 525 533 533 546 564 594 Soft (°C.) 785 743 750 761 784 816 CTE 62.2 66.6 74.1 81.4 87.6 94 Density2.323 2.38 2.426 2.414 2.467 2.5 2.525 Liquidus Temp 1240 1050 825 840855 730 730 Liq Visc (kP) 8 42 3802 2344 200255 1454433 200 P (° C.)1637 1595 1574 1550 1535 1549

The details on the role of each glass component will now be described.Silicate glasses with minimal levels or concentrations of NBOs show thehighest toughness and will occur at compositions at or nearB₂O₃=NBO=R₂O+R′O−Al₂O₃−ZrO₂.

The largest single constituent of the alkali aluminosilicate glass isSiO₂, which forms the matrix of the glass and is present in theinventive glasses in a concentration ranging from about 62 mol % up toand including about 70 mol %. SiO₂ serves as a viscosity enhancer thataids formability and imparts chemical durability to the glass. Atconcentrations that are higher than the range given above, SiO₂ raisesthe melting temperature prohibitively, whereas glass durability suffersat concentrations below the range. In addition, lower SiO₂concentrations can cause the liquidus temperature to increasesubstantially in glasses having high alkali or alkali earth oxideconcentrations.

The greater alkali metal oxide content facilitates melting, softens theglass, enables ion exchange, decreases melt resistivity, and breaks upthe glass network which increases thermal expansion and decreasesdurability. Mixtures of alkali metal oxides help depress the liquidustemperature and may enhance ion exchange as well. While Li₂O providesfast ion exchange, low density, and high modulus, it is also quiteexpensive. Na₂O is very desirable for ion exchange with K⁺ ions forchemical strengthening and makes stable glasses with respect todevitrification. Small amounts of K₂O relative to Na₂O actually helpincrease the rate of K⁺ for Na⁺ ion exchange and decrease liquidus, butalso increase the thermal expansivity of the glass.

Alumina (Al₂O₃) and, to a lesser extent, zirconia (ZrO₂) have theopposite effect of the alkali metal oxides. In addition, Al₂O₃ scavengesNBOs to form AlO₄ tetrahedra while making the glass thermally harder.Alumina and zirconia also provide lower expansion and greater durabilitybut, at high concentrations, make the glass more difficult to melt. Inmost ion exchangeable glasses, R₂O>Al₂O₃, since glasses in whichR₂O=Al₂O₃ are very difficult to melt unless B₂O₃ is present.

Alkaline earth oxides help create a steeper viscosity curve for theglasses. Replacing alkali metal oxides with alkaline earth metal oxidesgenerally raise the anneal and strain points of the glass while loweringthe melting temperatures needed to make high quality glass. MgO and CaOare less expensive than SrO and BaO and do not increase the density asmuch as the heavier oxides. BaO is also considered to be a hazardous ortoxic material, and its presence is therefore undesirable. Accordingly,in one embodiment, the glass is substantially free of barium. Largeamounts of MgO tend to increase the liquidus temperature, as the oxideis prone to form forsterite (Mg₂SiO₄) at low MgO concentrations insodium aluminosilicate glasses.

B₂O₃ may be used as a flux to soften glasses, making them easier tomelt. B₂O₃ also helps scavenge non-bridging oxygen atoms (NBOs), whichare created when the amount or concentration of modifiers exceeds thatof Al₂O₃. B₂O₃ converts the NBOs to bridging oxygen atoms through theformation of BO₄ tetrahedra, which increases the toughness of the glassby minimizing the number of weak NBOs. B₂O₃ also lowers the hardness ofthe glass which, when coupled with the higher toughness, decreases thebrittleness, thereby resulting in a mechanically durable glass.

In one embodiment, the alkali aluminosilicate glass described herein issubstantially free of lithium. As used herein, “substantially free oflithium” means that lithium is not intentionally added to the glass orglass raw materials during any of the processing steps leading to theformation of the alkali aluminosilicate glass. It is understood that analkali aluminosilicate glass or an alkali aluminosilicate glass articlethat is substantially free of lithium may inadvertently contain smallamounts of lithium due to contamination. The absence of lithium reducespoisoning of ion exchange baths, and thus reduces the need to replenishthe salt supply needed to chemically strengthen the glass. In addition,due to the absence of lithium, the glass is compatible with continuousunit (CU) melting technologies such as the down-draw processes describedabove and the materials used therein, the latter including both fusedzirconia and alumina refractories and zirconia and alumina isopipes.

In one embodiment, the silicate glass comprises at least one alkalimetal oxide and is ion exchangeable. As used herein, the term“ion-exchangeable” is understood to mean that the glass is capable ofbeing strengthened by ion exchange processes that are known to thoseskilled in the art. Such ion exchange processes include, but are notlimited to, treating the heated alkali aluminosilicate glass with aheated solution containing ions having a larger ionic radius than ionsthat are present in the glass surface, thus replacing the smaller ionswith the larger ions. Potassium ions, for example, could replace sodiumions in the glass. Alternatively, other alkali metal ions having largeratomic radii, such as rubidium or cesium, could replace smaller alkalimetal ions in the glass. Alternatively, the smaller alkali metal ionscould be replaced by Ag⁺ ions. Similarly, other alkali metal salts suchas, but not limited to, sulfates, halides, and the like may be used inthe ion exchange process. In one embodiment, the down-drawn glass ischemically strengthened by placing it a molten salt bath comprising KNO₃for a predetermined time period to achieve ion exchange. In oneembodiment, the temperature of the molten salt bath is about 430° C. andthe predetermined time period is about eight hours.

Down-draw processes produce surfaces that are relatively pristine.Because the strength of the glass surface is controlled by the amountand size of surface flaws, a pristine surface that has had minimalcontact has a higher initial strength. When this high strength glass isthen chemically strengthened, the resultant strength is higher than thatof a surface that has been a lapped and polished. Chemical strengtheningor tempering by ion exchange also increases the resistance of the glassto flaw formation due to handling. Down-drawn glass may be drawn to athickness of less than about 2 mm. In addition, down drawn glass has avery flat, smooth surface that can be used in its final applicationwithout costly grinding and polishing. In one embodiment, thedown-drawable glass has a liquidus viscosity of at least 100 kpoise. Inanother embodiment, the liquidus viscosity is at least 160 kpoise, and,in a third embodiment, the liquidus viscosity is at least 220 kpoise.

Surface compressive stress refers to a stress caused by the substitutionduring chemical strengthening of an alkali metal ion contained in aglass surface layer by another alkali metal ion having a larger ionicradius. In one embodiment, potassium ions are substituted for sodiumions in the surface layer of the glass described herein. The glass has asurface compressive stress of at least about 200 MPa. In one embodiment,the surface compressive stress is at least about 600 MPa. The alkalialuminosilicate glass has a compressive stress layer that has a depth ofat least about 30 μm and, in another embodiment, the depth of thecompressive stress layer is at least about 40 μm.

The replacement of smaller ions by larger ions at a temperature belowthat at which the glass network can relax produces a distribution ofions across the surface of the glass that results in a stress profile.The larger volume of the incoming ion produces compressive stress (CS)on the surface and tension in the center (CT) of the glass. Thecompressive stress is related to the central tension by the followingrelationship:CS=CT×(t−2DOL)/DOL;where t is the thickness of the glass and DOL is the depth of exchange,also referred to as depth of layer.

In addition to the elements described above, other elements andcompounds may be added to eliminate or reduce defects within the glass.The glasses of the present invention tend to exhibit 200 Poiseviscosities that are relatively high, between about 1500° C. and 1675°C. Such viscosities are typical of industrial melting processes, and insome cases melting at such temperatures may be required to obtain glasswith low levels of inclusions. To aid in eliminating gaseous inclusions,it may be useful to add chemical fining agents. Such fining agents fillearly-stage bubbles with gas, thus increasing their rise velocitythrough the melt. Typical fining agents include, but are not limited to:oxides of arsenic, antimony, tin and cerium; metal halides (fluorides,chlorides and bromides); metal sulfates; and the like. Arsenic oxidesare particularly effective fining agents because they release oxygenvery late in the melt stage. However, arsenic and antimony are generallyregarded as hazardous materials. Accordingly, in one embodiment, theglass is substantially free of antimony and arsenic, comprising lessthan about 0.05 wt % or about 0.05 mol % of each of the oxides of theseelements. Therefore, it may be advantageous in particular applicationsto completely avoid using arsenic or antimony, and instead using anontoxic component such as tin oxides, halides or sulfates to produce afining effect. Tin (IV) oxide (SnO₂) and combinations of tin (IV) oxidewith at least one of cerium (IV) oxide and halides are particularlyuseful as fining agents in the present invention.

The glasses of the present invention have properties which areunexpected in light of the prior art. While soda lime silica glasseshaving decreased brittleness have been previously described, suchglasses are neither fusion formable nor ion exchangeable to depths oflayer of 20 μm. Moreover, the glasses of the prior art are not as hardas the glasses described herein.

In addition to maximizing toughness, the silicate glasses describedherein also allow hardness to be maximized without increasingbrittleness. High levels of hardness provide greater scratch resistance.However, higher hardness increases brittleness unless toughness iscorrespondingly increased. Thus, if high levels of hardness can beaccompanied by increased toughness, the resulting glass will have bothincreased scratch resistance and low brittleness. The silicate glass isresistant to both chipping and scratching, making it well suited for usein cover plates, touch screens, watch crystals, solar concentrators,windows, screens, containers, and other applications which requirestrong and tough glass with good scratch resistance.

While typical embodiments have been set forth for the purpose ofillustration, the foregoing description should not be deemed to be alimitation on the scope of the invention. Accordingly, variousmodifications, adaptations, and alternatives may occur to one skilled inthe art without departing from the spirit and scope of the presentinvention.

What is claimed is:
 1. A silicate glass, the silicate glass being ionexchangeable and comprising: 62-70 mol % SiO₂; 0-18 mol % Al₂O₃; >0-10mol % B₂O₃; 0-15 mol % Li₂O; 6-20 mol % Na₂O; 0-18 mol % K₂O; >0-17 mol% MgO; and ZrO₂, wherein 16 mol %≦Al₂O₃ +B₂O₃ +ZrO₂≦29 mol %; 14 mol%≦R₂O +R′O≦25 mol %; and -15 mol %≦(R₂O +R′O—Al₂O₃—ZrO₂)—B₂O₃≦4 mol %,where R is at least one of Li, Na, K, Rb, and Cs, and R′ is at least oneof Mg, Ca, Sr, and Ba.
 2. The silicate glass according to claim 1,wherein the silicate glass is substantially free of at least one oflithium, antimony oxide, and arsenic oxide.
 3. The silicate glassaccording to claim 1, wherein the silicate glass comprises 0-18 mol %CaO.
 4. The silicate glass according to claim 1, wherein the silicateglass comprises >0-5 mol % ZrO₂.
 5. The silicate glass according toclaim 1, wherein the silicate glass comprises >0-2 mol % ZrO₂.
 6. Thesilicate glass according to claim 1, wherein the silicate glass has atoughness in a range from about 0.7 MPa m^(0.5) up to about 1.2 MPam^(0.5).
 7. The silicate glass according to claim 6, wherein thesilicate glass has a brittleness of less than about 8.5 μm^(−0.5). 8.The silicate glass according to claim 1, wherein the silicate glass hasa liquidus viscosity of at least 100 kpoise.
 9. The silicate glassaccording to claim 1, wherein the silicate glass is ion exchanged andhas a surface compressive stress of at least about 200 MPa and a surfacecompressive layer, the surface compressive layer having a depth of atleast about 30 μm.
 10. The silicate glass according to claim 1, whereinthe silicate glass forms a cover plate for a mobile electronic device,wherein the cover plate has a thickness in a range from about 0.3 mm upto about 1.5 mm.
 11. An electronic device comprising a cover platecomprising the silicate glass of claim
 1. 12. A silicate glass, thesilicate glass being ion exchangeable and comprising 62-70 mol % SiO₂;0-18 mol % Al₂O₃; >0-10 mol % B₂O₃; 6-20 mol % Na₂O, 0-15 mol % Li₂O;0-18 mol % K₂O; >0-17 mol % MgO; and SnO₂, wherein 16 mol %≦Al₂O₃+B₂O₃+ZrO₂≦29 mol %; and −15 mol %≦(R₂O +R′O—Al₃O₃ —ZrO₂)—B₂O₃≦4 mol %, whereR is at least one of Li, Na, K, Rb, and Cs, and R′ is at least one ofMg, Ca, Sr, and Ba.
 13. The silicate glass according to claim 12,wherein the silicate glass is substantially free of at least one oflithium, antimony oxide, and arsenic oxide.
 14. The silicate glassaccording to claim 12, wherein the silicate glass comprises 0-18 mol %CaO.
 15. The silicate glass according to claim 12, wherein the silicateglass comprises >0-5 mol % SnO₂.
 16. The silicate glass according toclaim 12, wherein 14 mol %≦R₂O +R′O≦25 mol %, where R is at least one ofLi, Na, K, Rb, and Cs, and R′ is at least one of Mg, Ca, Sr, and Ba. 17.The silicate glass according to claim 12, wherein the silicate glass hasa toughness in a range from about 0.7 MPa m^(0.5) up to about 1.2 MPam^(0.5).
 18. The silicate glass according to claim 12, wherein thesilicate glass has a brittleness of less than about 8.5 μm^(−0.5). 19.The silicate glass according to claim 12, wherein silicate glass has aliquidus viscosity of at least 100 kpoise.
 20. The silicate glassaccording to claim 12, wherein the silicate glass forms a cover platefor a mobile electronic device, wherein the cover plate has a thicknessin a range from about 0.3 mm up to about 1.5 mm.
 21. An electronicdevice comprising a cover plate comprising the silicate glass of claim12.